The answer to the question in the title is succinctly given by Niv Dror: “Once an app becomes significant enough to pose a threat to the big players, they either get acquired or significantly handicapped by a competitive feature or restricted access.” This we might call 'normal ecosystem' (styled after Kuhn's 'normal science'). Eventually there will be sufficient dissonance in the normal to generate what we might call an 'ecosystem revolution'. But what's key is that the new ecosystem will be incommensurate with the existing ecosystem. Instead of depending on Facebook and Twitter and the rest it will replace them.